Given the haunted reputation of Inverlochy House, it would be easy to attribute this to a spiritual presence. It even looks a little like the green 'Slimer' ghost in Ghostbusters - Wikia article
But this orb is definitely caused by lens flare. The sun was shining from around the corner of the building, directly into my camera lens. The lens was stopped down to f5.6, and you can see the hint of the hexagonal shape formed by the lens aperture blades.
Still, it's possible that some orbs in photos have a paranormal reason for being. It would seem more likely that you'd be able to see those ones with your own eyes, not just in a photograph. But there is much we still have to learn. Take nothing for granted!
Orb page on Strange Occurrences site
Lens flare examples on Strange Occurrences site
The Haunting of Inverlochy - video
A tour through Wellington's haunted house - TV3 news story
Lens flare causing false-positives in paranormal investigation